Scintillation crystal radiation detector which uses a multiwire counter structure in a position sensitive photo-multiplier

ABSTRACT

A PET radiation detector includes a pair of amplifying gaps 3,21 to enhance the electron yield from a TMAE gas, BaF 2  gamma detector, together with a gate electrode 17 to inhibit passage of charge to the detector electrodes and reverse passage of ions to the crystal. A further reverse-biased gap may be positioned adjacent the crystal to prevent charge build-up thereon. Shield electrodes 23,25 prevent gate switching signals causing spurious responses in the detector circuit.

BACKGROUND OF THE INVENTION

This invention relates to detectors and, in particular, to detectors foggamma radiation.

1. FIELD OF THE INVENTION

2. DESCRIPTION OF RELATED ART

Referring now to the drawings, FIG. 1 shows a prior art design. Incidentgamma radiation causes a BaF₂ crystal 1 to scintillate, generatingultra-violet (uv) photons. The UV photons convert in a gas space 3adjacent to the crystal and the resulting handful of electrons areamplified in a high electric field applied between a conductive mesh 5on the crystal surface and the cathode 7 of a multiwire proportionalcounter (MWPC) 9. The signal is transferred into this section andfurther amplified on the anode wires 11. Some form of read-out is builtinto the MWPC section.

We found that this structure was very unstable and started to breakdownafter 20 minutes or so, due to the charging of the crystal surface bythe positive, ions returning from the avalanche in the MWPC. However, weattempted to address this instability by installing a protective gap 13(FIG. 2) with a reverse bias immediately against the crystal face. Thisgap (preferably 0.5-1.0 mm wide) sacrifices a little signal for a verymuch enhanced stability. He have found that with 100 V of reverse biasthe modified counter will run all day without showing charging effects.

In the prior art positron camera a severe practical problem is caused bythe very high ratio of single counts to coincidence (i.e. useful) counts(up to ≈50:1). This overloads the gain elements of the MWPC and causesserious deadtime losses in the read-out system. He found two furthermodifications which improve this situation significantly. Firstly, theinitial parallel amplifying gap 3 which now follows the crystal barriergap is separated from the MWPC by a wide gap 19 (˜30 mm). (This on itsown further enhances the stability of the counter.) In order to do ourfast coincidence we would now like to take a trigger signal from theback of this gap. However, as this would demand too much gain from onegap we insert a further gap 21 and take the trigger signal from its rearface.

Secondly the fast coincidence with the other detector is performed whilethe electron cloud drifts towards a MWPC 9 which delivers a final burstof gain and performs the read-out. Roughly in the middle of the driftregion 15 is an electronic gate 17 operated by the coincidence circuit.This ensures that only "good" events trigger the MWPC and the read-outsystem. This simultaneously enhances the stability of the counter anddramatically reduces the pile-up in the read-out electronics. This gatehas been carefully designed with shield electrodes 23,25 to minimise theinterference it can cause in the read-out electronics.

With these modifications our tests to date have been able to demonstratea quantum efficiency of 20% and a spatial resolution of 6 mm fwhm with atime resolution of 4 ns fwhm. The efficiency is three times that of thelead system and the time resolution 1/3. This means a factor of 9 insensitivity and a factor of 27 in signal to noise ratio. The predictedmaximum data rate rise is from 2 kHz to 20 kHz under comparableconditions.

SUMMARY OF THE INVENTION

In our new radiation detector, 511 keV gamma rays are converted into UVphotons which are then detected in TMAE vapour in a multiwire counterstructure which functions as a position sensitive photomultiplier.

According to the present invention there is provided a radiationdetector comprising a scintillation crystal, means to convert opticalradiation into electrical charge carriers and detector means to detectthe generated charge carriers wherein a gap provided with means toinhibit the passage of charge carriers is positioned between saidcrystal and said detector means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be particularly described, by way of example, withreference to the following accompanying drawings:

FIG. 1 shows a schematic section of a prior art positron camera;

FIG. 2 shows a schematic layout of a camera in accordance with aspecific embodiment of the present invention;

FIG. 3 is a section through a practical embodiment of the invention; and

FIGS. 4 and 5 show details of the apparatus of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A practical embodiment of the detector is shown in FIG. 3. Thiscomprises a barium fluoride crystal detector 31 comprising twelve tilesof BaF₂ mounted on a stainless steel frame 33. Next there is a series ofwire planes. The first plane 35 consists of 50 μm diameter wire at apitch of 500μm and is spaced 0.5 mm from the BaF₂ crystal. The secondplane 37 consists of 100 μm diameter wire at 1 mm pitch and is spaced3.0 mm from the first plane. The third plane 39 also consists of 100 μmdiameter wire at 1 mm pitch and is spaced 9.0 mm from the second plane.A gate 41 comprising 100 μm wires at 1 mm pitch is positioned 20.0 mmfrom the third wire plane and has first and second metallic mesh screens43,45 positioned one on either side. Following a further gap of 13.2 mm,Is a back end detector 47 which looks like a conventional multi-wireproportional counter with orthogonal cathodes. The cathodes consist of50 μm wire at 2.0 mm pitch and the anode/cathode plane comprises 20 μmanode wires and 100 μm cathode wires at 4.0 mm pitch. X-andY-coordinates are obtained with delay lines 49,51. The detector ismounted within a sealed enclosure comprising aluminiumhoneycomb-structure sheets 53 mounted on a stainless steel frame 55.Heater pads are provided to keep the enclosure at a constant temperatureof 60° C. at which the TMAE vapour pressure is 4.5 mB. The honeycombaluminium sheet Is structurally strong but is substantially transparentto the gamma photons.

In operation, a 511 KeV gamma photon is trapped by the BaF₂ crystalwhich emits a flash of 190 nm ultra-violet radiation. The UV radiationis absorbed by TMAE gas within the camera chamber and is photo-ionisedcreating electrons.

A voltage source V1 is connected between the first and second planes,creating a field of 300 V/mm which permits the creation of furtherelectrons. A gain of a level generated by this field in a wider gapwould be unstable, so a second amplification gap connected to a furthervoltage source V2 has a lower field strength of about 150 V/mm. The twogaps will together provide sufficent electrons for a detectable signal.

With a positron camera, there are two detectors, one on either side ofthe source. The positron emission event is characterised by coincidenceof signals in the two detectors. A signal is therefore taken from thethird plane and fed into an amplifier A1 and discriminator D. The signalfrom the corresponding amplifier A2 on the other detector is similarlyextracted and examined for coincidence. When events are detected by boththird, planes within a time window of 5 ns, the two events, one on eachend are in time coincidence.

Between the third plane and the multi-wire detectors is placed a gateelectrode which is normally biased ±20 V on alternate wires. It is aflat plane of wires, but with alternate wires connected to two bus barsso that if one wire is up, the next wire is down. As long as thisvoltage Is applied, the gate acts as a barrier to the passage ofelectrons between the third plane and the detectors. When coincidenceoccurs, the gate electrodes are brought to zero volts whilst thetriggering electrons are still in process of passing through the driftfield between the third plane and the gate. A window of about 200 ns Isavailable for this decision and action.

When the bias is removed from the gate wires, electrons can pass andwill drift to the multi-wire and give rise to a normal avalanche aroundan anode wire, induce a signal on the cathode, thus permitting a readoutof the X- and Y-coordinates.

The effect of the gating Is that the back end of the counter operatesonly at the coincidence rate whereas the front end generates electronsat a much higher rate corresponding to single events. This is animprovement of greater than 100:1, which has a corresponding effect onthe signal/noise ratio of the detected signal. It also relievescongestion at the detector.

Another advantageous effect of the gate electrode is that it also actsas a barrier to the reverse transmission of positive ions from themulti-wire region.

Yet a further advantage of this arrangement is that the actualgeneration of positive Ions Is reduced since a large signal Is producedonly after coincidence has opened the gate rather than continuously aswith a conventional arrangement.

A problem with the fast switching of the gate electrode is that, withthe level of signal and impedance of the multi-wire detector, spurioussignals could be Induced In the detector circuit. To prevent this acopper mesh screen Is provided on either side of the gate, so the gateelectrode. The screens are connected to potential sources appropriate tomaintain the electron drift field. A metallic shield on the framecompletely encloses the gate. AC continuity Is maintained by surfacemount capacitors bridging connecting gaps, thereby creating a shieldingcage round the gate electrode.

A further problem Is that of positive ions drifting back to the frontend, since BaF₂ Is an almost perfect Insulator. This problem isaddressed by two measures. Firstly a metallic wire is wound round thecrystal. With 25 μm wire at 250 μm pitch there is still 90% spare area,but there is an electrode to trap the ions and also to reduce the lengthof the discharge track enormously.

Secondly, in the front of this crystal is provided a reverse blased gap0.5 mm wide. so that the positive ions are not driven to the crystal.When they arrive at the first plane, 0.5 mm in front of the crystal,that Is the most negative point and they don't go any further. Thedisadvantage is that any UV light which is converted In the first 0.5 mmgap is effectively lost.

The thickness of the BaF₂ crystal may be Increased to improvesensitivity. This is, however, a trade-off against resolution with apractical maximum of 16 mm.

We claim:
 1. A radiation detector comprising a scintillation crystal,means to convert optical radiation into electrical charge carriers anddetector means to detect the generated charge carriers wherein a gapprovided with means to inhibit the passage of charge carriers ispositioned between said crystal and said detector means.
 2. A radiationdetector as claimed in claim 1 wherein said means to inhibit the passageof charge carriers comprises a grid immediately adjacent saidscintillation crystal and a reverse bias applied to said grid.
 3. Aradiation detector as claimed in claim 2 wherein said grid is spaced 0.5to 1.0 mm from said scintillation crystal.
 4. A radiation detector asclaimed in claim 3 wherein said bias is of the order of 100 V.
 5. Aradiation detector as claimed in claim 1 wherein it includes asuccession of charge amplifying gaps across which different fields areapplied.
 6. A radiation detector as claimed in claim 5 wherein itincludes a pair of charge amplifying gaps the first of which is of theorder of 3 millimeters in width and the second of which is of the orderof 9 millimeters in width.
 7. A radiation detector as claimed in claim 6wherein fields of the order of 300 V/mm and 150 V/mm are applied acrossrespective said gaps.
 8. A radiation detector as claimed in claim 1wherein said means to inhibit the passage of charge carriers comprises agate electrode placed intermediate said crystal and said detector means.9. A method of detecting radiation comprising using a scintillationcrystal to generate optical radiation, using said optical radiation tocreate an electrical charge by means of a photo-ionising medium,multiplying said charge in an electrical field, causing said multipliedcharge to drift across a gap between field-creating electrodes, derivinga signal from said charge carriers and using said signal to control agate electrode positioned In the drift path of said charge carriers.